Color cathode ray tube having shadow mask with prescribed bridge widths

ABSTRACT

A shadow mask opposed to a phosphor screen has a substantially rectangular effective surface (30) where slit-like apertures are formed. The apertures are disposed so as to constitute a plurality of aperture rows which extend in parallel with the short axis of the effective surface and are disposed in the long axis of the effective surface. Each of the aperture rows includes a plurality of aperture, and bridges (38) positioned between any adjacent pair of the apertures. The width B of the bridges in the lengthwise direction of the aperture rows, positioned an intermediate between the short axis of the effective surface and a short side edge thereof is greater than that of the bridges positioned at a peripheral portion of the effective surface.

TECHNICAL FIELD

The present invention relates to a color cathode ray tube andparticularly to a color cathode ray tube comprising a shadow mask havinga number of apertures.

BACKGROUND ART

In general, a color cathode ray tube comprises a vacuum envelope havinga face panel, a phosphor screen formed on an inner surface of the facepanel and including three color phosphor layers capable of radiating inblue, green, and red, a shadow mask opposed to the phosphor screen, andan electron gun provided in a neck of the vacuum envelope. The shadowmask includes a mask body having a number of apertures for passingelectron beams, and a mask frame supporting the peripheral edge portionof the mask body. In this color cathode ray tube, three electron beamsemitted from the electron gun scan the phosphor screen through theshadow mask, thereby displaying a color image.

The shadow mask is provided to select the three electron beams to berespectively landed on predetermined positions on the three colorphosphor layers, and this selection must be correctly carried out suchthat three electron beams are respectively landed correctly onpredetermined positions of the three color phosphor layers, in orderthat a color image displayed on the phosphor screen obtains an excellentcolor purity. Therefore, the shadow mask must be arranged so that apredetermined positional relationship is always maintained with respectto the phosphor screen during operation of the color cathode ray tube,i.e., the distance (q value) between the shadow mask and the phosphorscreen must always fall within a predetermined tolerance range.

However, in a color cathode ray tube of a shadow mask type, only 1/3 orless of the entire electron beams emitted from the electron gun reachthe phosphor screen, and the other remaining beams collide onto theshadow mask. Further, the shadow mask is heated by those collidingelectron beams and expands towards the phosphor screen, i.e., so-calleddoming occurs. The doming can be divided into two types.

One type that occurs is at the beginning of starting operation of acolor cathode ray tube. Specifically, at the starting operation, themask body of the shadow mask is mainly heated and a temperaturedifference occurs between the mask body and the mask frame which isprovided on the peripheral edge portion of the mask body. Due to thetemperature difference, doming occurs.

The other type that occurs is locally in a relatively short time when animage having a high luminance is locally displayed and the mask body isthereby locally heated and expanded.

Once doming of a shadow mask occurred, the position of the shadow maskrelative to the phosphor screen changes and the q value derives from thetolerance range. Landing positions of electron beams with respect to thephosphor layers are then dislocated from predetermined positions, and asa result, the color purity of an image displayed is degraded. Landingdislocations thus caused by doming vary depending on the position of animage pattern to be displayed, the luminance thereof, and thecontinuation time of a high-luminance image pattern.

In addition, a landing dislocation of an electron beam caused by localdoming when an image having a high luminance is displayed locally tendsto easily occur at an intermediate region between the center of theshadow mask and an end of the horizontal axis thereof. This can beassociated with doming of the shadow mask and the deflection angle of anelectron beam. For example, even when doming occurs in the vicinity ofthe vertical axis of a shadow mask, the deflection angle of electronbeams is small within this portion, so that the electron beam is notmuch affected by doming and a landing dislocation caused therefrom issmall. Meanwhile, the peripheral portion of the mask body is supportedon the mask frame which has a large heat capacitance by a non-apertureportion, so that heat in the mask body diffuses into the mask frame evenwhen the peripheral portion of the mask body is locally heated.Therefore, doming which occurs in the peripheral portion of the maskbody is of a low level and causes only a small landing dislocation.

In contrast, in an intermediate region between the center of the shadowmask and each end of the horizontal axis thereof, electron beams have alarge deflection angle, and doming of a high level occurs when theshadow mask is locally heated within these intermediate regions. As aresult, a landing dislocation tends to occur most easily at thoseportions of the phosphor layer which face the intermediate regions ofthe shadow mask.

In order to prevent a local heat expansion of a shadow mask and toprevent color blurring, the curvature of a shadow mask in its horizontalcross-section should be enlarged. In recent years, however, it has beena main trend to use a color cathode ray tube having a flattened facepanel, and accordingly, such a cathode ray tube has a flattened shadowmask. Therefore, it is difficult to restrict local doming which occursin a relatively short time and to eliminate a landing dislocation, onlyby means of enlarging the curvature of the shadow mask in its horizontalcross-section.

In a television set incorporating a color cathode ray tube, a landingdislocation occurs when a vibration caused by sounds or voices from alaud speaker during operation of the television set is transferred tothe color cathode ray tube, the mask body itself vibrates (or causeshowling) and causes a landing dislocation of electron beams, in additionto a landing dislocation caused due to doming of the shadow mask asdescribed above. Therefore, such a landing dislocation caused by howlingmust be restricted.

Since the peripheral edge portion of a shadow body is fixed to a maskframe, a vibration has a small amplitude in this portion. However, inthe intermediate regions of the mask body as described above, thevibration is large and a landing dislocation has the largest amount.

DISCLOSURE OF INVENTION

The present invention has been made in view of the above problem and itsobject is to provide a color cathode ray tube capable of reducing localdoming and vibration of a shadow mask and hinders color blurring.

In order to achieve the above object, a color cathode ray tube accordingto the present invention comprises a face panel having a substantiallyrectangular effective portion which has an inner surface of a curvedsurface and long and short axes perpendicular to each other; a phosphorscreen formed on the inner surface of the face panel and having a numberof phosphor layers each having a stripe-like shape extending in adirection along the short axis; and a shadow mask opposed to thephosphor screen and having a curved shape corresponding to the innersurface of the face panel.

The shadow mask includes a substantially rectangular effective surfaceprovided with a number of apertures for passing electron beams, havinglong and short axes respectively corresponding to the long and shortaxes of the face panel, and consisting of first and second halves whichare symmetric with the short axis, and a non-aperture portion positionedaround a periphery of the effective surface. The apertures are disposedso as to constitute a plurality of aperture rows extending in parallelwith the short axis and disposed in a direction of the long axis, eachof the aperture rows including a plurality of apertures disposed in adirection parallel to the short axis and bridges positioned between anyadjacent pair of the apertures.

A width of bridges in the direction of the short axis, which arepositioned at a substantially central region in each of the first andsecond halves is greater than a width of bridges in the direction of theshort axis, which are positioned at a peripheral portion of theeffective surface.

The bridges are formed so as to satisfy relations of: BMH>BH, BMH>BD,and BMH>ML, where BO is a width of the bridges in the direction of theshort axis, positioned at a center O of the effective surface of theshadow mask, BV is a width of the bridges in the direction of the shortaxis, positioned at end portions of the short axis, BH is a width of thebridges in the direction of the short axis, positioned at end portionsof the long axis, BD is a width of the bridges in the direction of theshort axis, positioned at end portions of diagonal axes, BMH is a widthof the bridges in the direction of the short axis, positioned at asubstantially central region of each of the first and second halves, andBML is a width of the bridges in the direction of the short axis,positioned at an intermediate portion between the short axis and each ofshort side edges the effective surface in parallel with the short axisand at near a long side edge of the effective surface in parallel withthe long axis.

A width B (x, y) of a bridge at a given coordinate position on theeffective surface is formed to be a size expressed by aquaternary-exponential polynominal as follows: ##EQU1## where the longaxis of the effective surface of the shadow mask is an x-axis, the shortaxis thereof is a y-axis, and c is a coefficient.

According to a color cathode ray tube having a structure constructed asdescribed above, the width of a bridge in the short axis direction,positioned at a substantially central portion of each of the first andsecond halves of the effective surface is greater than the width ofbridges in the short axis direction, positioned at a peripheral portionof the effective surface. Therefore, the heat capacitance and therigidity of the shadow mask is greater at the central portions of thefirst and second halves of the effective surface of the shadow mask thanat the peripheral portion.

Therefore, the doming amount at the central portions of the effectivesurface where doming tend to occur most easily can be reduced anddegradation of the color purity caused by doming can be restricted. Atthe same time, when the color cathode ray tube vibrates, a vibration ofthe central portions of the first and second halves of the effectivesurface can be reduced, so that degradation of the color purity causedby a vibration can be reduced.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1 to 5 show a color cathode ray tube according to an embodiment ofthe present invention, in which:

FIG. 1 is a longitudinal sectional view of the color cathode ray tube,

FIG. 2 is a plan view showing the inner side of a face panel of thecolor cathode ray tube,

FIG. 3 is a plan view showing a shadow mask of the color cathode raytube,

FIG. 4 is an enlarged plan view showing the shadow mask of the colorcathode ray tube, and

FIG. 5 is a cross sectional view taken along a line V--V in FIG. 4;

FIG. 6 is a graph showing a relationship between the width of a bridgeand the distance from the vertical axis; and

FIG. 7 is a graph showing the X-Y coordinate position of an effectivearea of the shadow mask.

BEST MODE OF CARRYING OUT THE INVENTION

In the following, a color cathode ray tube according to an embodiment ofthe present invention will be described in details with reference to theaccompanying drawings.

As shown in FIGS. 1 and 2, the color cathode ray tube comprises a vacuumenvelope 10 made of glass. The vacuum envelope 10 includes a face panel3 having a substantially rectangular effective portion 1 and a skirtportion 2 provided on the peripheral portion of the effective portion, afunnel 4 connected with the skirt portion 2, and a cylindrical neck 7projecting from the funnel 4.

The effective portion 1 has a substantially rectangular shape having ahorizontal axis (or long axis) X and a vertical axis (or short axis)perpendicular to each other, extending through a tube axis Z of thecathode ray tube. In addition, the inner surface of the effectiveportion 1 is formed of a concave curved surface which is not spherical.On the inner surface of the effective portion 1 is formed a phosphorscreen 5 which includes three color phosphor layers 20B, 20G, and 20Rrespectively capable of radiating in blue, green, and red, and lightshield layers 23 provided between the phosphor layers. The phosphorlayers 20B, 20G, and 20R are formed like stripes extending in parallelwith the vertical axis Y and disposed one after another in the X-axisdirection.

Also, in the vacuum envelope 10, a shadow mask 21 having a substantiallyrectangular shape corresponding to the phosphor screen 5 is arranged toface the phosphor screen 5. The shadow mask 21 comprises a substantiallyrectangular mask body 27 having a number of apertures 25 and arectangular mask frame 26 supporting the peripheral edge portion of themask body. The shadow mask 21 is supported on the face panel 3 in amanner in which elastic support members 15 each having a substantiallywedge-like shape and fixed on side walls of the mask frame 26 areengaged with stud pins 16 projecting from the inner surface of the skirtportion of the face panel 3. In this manner, the mask body 27 is opposedto the phosphor screen 5 with a predetermined distance therebetween.

An electron gun 9 for emitting three electron beams 8B, 8G, and 8R whichpass in one same plane is provided in the neck 7.

In the color cathode ray tube constructed in a structure as describedabove the three electron beams 8B, 8G, and 8R emitted from the electrongun are deflected by horizontal and vertical magnetic fields generatedby a deflection yoke 11 attached outside the funnel 4, and scan thephosphor screen 5 through the shadow mask 21, thereby displaying a colorimage.

As shown in FIGS. 3 and 4, the mask body 27 is formed by processing athin metal plate having a thickness of 0.10 to 0.30 mm, and has asubstantially rectangular effective surface 30 in which a number ofslit-like apertures 25 are formed for passing electron beams, and anon-aperture portion 32 positioned around the periphery of the effectivesurface and having no apertures. The mask body 27 has a center O where atube axis Z passes, and a horizontal (or long) axis X and a vertical (orshort) axis Y which are perpendicular to each other and passing thecenter O. Also, the mask body 27 is formed as a curved surfacecorresponding to the inner surface of the effective portion 1. Theeffective surface 30 consists of first and second halves 30a and 30bwhich are symmetric with the vertical axis Y. The non-aperture portion32 is fixed to the mask frame 26.

A number of slit-like apertures 25 are arranged so as to constitute aplurality of aperture rows R which extend in parallel to the verticalaxis Y and are disposed at a predetermined pitch PH in the direction ofthe horizontal axis X. Each of the aperture rows R includes a pluralityof apertures 25 disposed at a predetermined pitch PV in the direction ofthe vertical axis Y with a bridge 38 being interposed between twoadjacent apertures 25.

As shown in FIGS. 4 and 5, each of the apertures 25 is defined by aboundary between a large aperture 25a opened to the surface facing thephosphor screen 5 and a small aperture 25b opened to the surface facingthe electron gun, in the mask body.

In the shadow mask 25 according to the present embodiment, the width Bof a bridge 28 provided between two adjacent apertures 25 disposed inthe direction of the vertical axis Y varies depending on its position onthe mask body 27. More specifically, in FIG. 6, a curve 41 indicates arelationship between the width B of bridges near the apertures 25disposed on the horizontal axis X of the mask body 27 and a distance tothe bridge from the vertical axis Y of the mask body 27, and a curve 42indicates a relationship between the width B of bridges disposed in thevicinity of each long side edge of the mask body 27 and a distance tothe bridge from the vertical axis Y.

As shown in FIG. 7, within the effective surface 30 of the mask body 27,a plurality of bridges 38 are formed so as to satisfy the followingrelations, where BO is the width of bridges 38 in the direction of thevertical axis Y, positioned at the center O of the effective surface 30,BV denotes the width of bridges 38 in the direction of the vertical axisY, positioned each end portion of the vertical axis Y, BH denotes thewidth of bridges 38 in the direction of the vertical axis Y, positionedat each end portion of the horizontal axis X, BD is the width of bridges38 in the direction of the vertical axis Y, positioned at each endportion of diagonal axes D, BMH denotes the width of bridges 38 in thedirection of the vertical axis Y, positioned at a central region 31a(see FIG. 3) of each of the first and second halves 30a, 30b, i.e., atan intermediate region between the vertical axis Y and one of the shortside edges of the effective surface 30 and between a pair of long sideedges of the effective surface, and BML is the width of bridges 38 inthe direction of the vertical axis Y, positioned at an intermediateportion between the vertical axis Y and a short side edge of theeffective surface on the long side edge of the effective surface.

BMH>BH

BMH>BD and

BMH>BML

Thus, the width BMH of the bridges 38 positioned at each of the firstand second central regions 31a and 31b is greater than the widths of thebridges in the other portions.

According to the shadow mask 21 constructed as described above, thepitch PV of apertures 25 disposed in the vertical direction is uniformover the entire effective surface 30, and the apertures 25 have aconstant width W in the direction of the horizontal axis X. Therefore,the area of each aperture 25 decreases as the width B of the bridge 38increases. However, if the bridges 38 positioned at the central regions31a and 31b are formed to have a large width B, the heat capacitance atthe central regions 31a and 31b of the first and second halves 30a and30b of the mask effective surface 30 can be increased to be greater thanthat of another portion such as the peripheral portion of the effectivesurface 30.

As a result, according to the shadow mask 21 as described above, evenwhen an electron beam having a high current density collides into thecentral regions 31a, and 31b on the mask effective surface 30 wheredoming tends to occur easily and the central regions 31a and 31b arethereby heated, a temperature increase thereby caused in these regionscan be reduced since the central regions 31a and 31b have a large heatcapacitance. Further, even when a heat is transferred from the centralregions 31a and 31b to the peripheral portion of the effective surface30, the area of the peripheral portion has a small heat capacitance andcauses a large temperature increase, resulting in that a peak of thetemperature difference between each central region and the peripheralportion of the effective surface 30 can be reduced. Accordingly, localdoming of the mask body 27 which occurs with in a short time period canbe reduced and a landing dislocation caused by such local doming can bereduced. As a result, degradation of the color purity caused by alanding dislocation can be reduced, so that excellent image display isrealized.

The bridge width B of the shadow mask 21 can be easily realized by thefollowing polynominal. Specifically, the width B (x, y) of a bridge inthe direction of an aperture row at given coordinates (x, y) on theeffective surface can be set by a quaternary-exponential polynominalrelating to x and y as follows, where c is a coefficient in an x-ycoordinate system defined by two perpendicular axes of the horizontalaxis X and the vertical axis Y passing the center of the effectivesurface 30. ##EQU2##

The width B of a bridge 38 set by the above polynominal is, for example,arranged as follows in case of a shadow mask for a 28-inch color cathoderay tube.

The bride width BO at the center O of the mask:

BO=0.160 mm

The bridge width BMH at an intermediate portion on the horizontal axis:

BMH =0.160 mm

The bridge width BH at an end portion of the horizontal axis X:

BH=0.130 mm

The bridge width BV at an end portion of the vertical axis Y:

BV=0.140 mm

The bridge width BML at an intermediate portion on a long side edge:

BML=0.125 mm

The bridge width BD at an end portion on a diagonal axis D:

BD=0.140 mm

The coefficient c is selected as follows.

c0=1.600000×10⁻⁰¹

c1=4.175079×10⁻⁰⁷

c2=-1.181269×10⁻¹¹

c3=-6.110379×10⁻⁰⁷

c4=-6.407131×10⁻¹¹

c5=1.082887×10⁻¹⁵

c6=-1.219065×10⁻¹¹

c7=3.618716×10⁻¹⁶

c8=-1.471625×10⁻²¹

Accordingly, the area of slit-like apertures 25 in the first and secondcentral regions is smaller by 10% than that at the peripheral portion ofthe mask body, and the heat capacitance at the first and second centralregions can be greater by a corresponding amount than that at theperipheral portion. As a result, when an image having a high luminanceis displayed locally, doming can be reduced at the first and secondcentral regions where local doming tends to occur easily. At the sametime, doming caused by a temperature difference between the mask bodyand the mask frame provided in the peripheral portion thereof in thebeginning of starting operation of a color cathode ray tube can bereduced, so that the distance (q-value) between the shadow mask and thephosphor screen can be maintained within a predetermined range.Therefore, degradation of the color purity caused by a landingdislocation of electron beams with respect to three color phosphorlayers can be reduced. In particular, a remarkable advantage can beobtained in a color cathode ray tube in which the face panel isflattened and the effective surface of the shadow mask is accordinglyflattened, so that projection onto the outer surface of the face panelprovides an image with a natural appearance.

In addition, the width of a bridge is increased, the rigidity of thecurved surface of the mask body is improved. Therefore, by setting thewidth of the bridges in the first and second central regions of the maskbody to be larger than that in the peripheral portion of the mask body,the rigidity of the mask body can be relatively high at the first andsecond central regions in comparison with the peripheral portion of themask. Accordingly, even when a vibration is applied to the color cathoderay tube by a sound or voice from a loud speaker of a television set,the amplitude of the vibration is reduced at the intermediate portion ofthe mask. Meanwhile, the peripheral portion of the mask effectivesurface is in contact with non-aperture portion or a mask frame having ahigh rigidity and is therefore tends to less vibrate. As a result ofthis, the anti-vibration characteristic is improved over the entiremask, and degradation of an image due to a vibration of a shadow maskcan be reduced.

As has been described above, the area of a slit-like aperture is changedby changing the bridge width, and accordingly, the radiation area ofthree color phosphor layers is changed in accordance with the area of aslit-like aperture, thereby effecting the luminance of the screen.However, the effective portion of the face panel generally is thicker ata peripheral portion thereof than at a center portion thereof. Inparticular, a face panel of a dark tint type used for improving thecontrast tends to have a low luminance at a peripheral portion of thescreen. Therefore, if the bridge width is set to be large at first andsecond central regions of the effective surface of the shadow mask, theluminance at the peripheral portion of the screen is relativelyincreased and the luminance becomes uniform over the entire screen area,resulting in no problems.

What is claimed is:
 1. A color cathode ray tube comprising:a face panelincluding a substantially rectangular effective portion which has aninner surface of a curved surface and long and short axes perpendicularto each other; a phosphor screen folded on the inner surface of the facepanel and having a number of phosphor layers each having a stripe-likeshape extending in a direction in parallel to the short axis; and ashadow mask opposed to the phosphor screen and having a curved shapecorresponding to the inner surface of the face panel, the shadow maskincluding a substantially rectangular effective surface provided with anumber of apertures for passing electron beams and having long and shortaxes respectively corresponding to the long and short axes of the facepanel, and first and second halves which are symmetric with the shortaxis, and a non-aperture portion located around a periphery of theeffective surface; the apertures being disposed so as to constitute aplurality of aperture rows extending in parallel with the shots axis anddisposed in a direction of the long axis, each of the aperture rowsincluding a plurality of the apertures disposed in a direction parallelto the short axis and bridges positioned between any adjacent pair ofthe apertures, and a width (BMH) of the bridges in the direction of theshort axis, which are positioned at a substantially central region ineach of the first and second halves, being greater than a width of thebridges in the direction of the short axis, which are positioned at theperipheral portion of the effective surface, wherein the differencebetween the width (BMH) and a width (BML) of the bridges in thedirection of the short axis, positioned at an intermediate portionbetween the short axis and a short side edge of the effective surfaceand near a long side edge of the effective surface in parallel with thelong axis, being larger than the difference between the width (BMH) anda width of any other bridges in the direction of the short axis, whichare positioned at the peripheral portion of the effective surface.
 2. Acolor cathode ray tube according to claim 1, wherein the bridges areformed so as to satisfy relations of: BMH>BH, BMH>BD₂ BMH>BML, andBMH-BML>BMH-BH or BMH-BD, where BO is a width of the bridges in thedirection of the short axis, positioned at a center O of the effectivesurface, BV is a width of the bridges in the direction of the shortaxis, positioned at each end portion of the short axis, BH is a width ofthe bridges in the direction of the short axis, positioned at each endportion of the long axis, BD is a width of the bridges positioned ateach end portion of diagonal axes of the effective surface, BMH is awidth of the bridges in the direction of the short axis, positioned ateach of the central regions of the first and second halves, and BML is awidth of the bridges in the direction of the short axis, positioned atan intermediate portion between the short axis and a short side edge ofthe effective surface and near a long side edge of the effective surfaceIn parallel with the long axis.
 3. A color cathode ray tube according toclaim 1, wherein a width B (x, y) at a given coordinate position on theeffective surface is formed to be a size expressed by aquaternary-exponential polynominal as follows: ##EQU3## where the longaxis of the effective surface of the shadow mask is an x-axis, the shortaxis thereof is a y-axis, and c is a coefficient.
 4. A color cathode raytube according to claim 1, wherein the plurality of apertures in each ofthe aperture rows are disposed at a predetermined pitch.
 5. A colorcathode ray tube according to claim 1, wherein each of the apertures hasa slit-like shape extending in the direction of the short axis.
 6. Acolor cathode ray tube comprising:a face panel including a substantiallyrectangular effective portion which has an inner surface of a curvedsurface and long and short axes perpendicular to each other; a phosphorscreen folded on the inner surface of the face panel and having a numberof phosphor layers each having a stripe-like shape extending in adirection in parallel to the short axis; and a shadow mask opposed tothe phosphor screen and having a curved shape corresponding to the innersurface of the face panel, the shadow mask including a substantiallyrectangular effective surface provided with a number of apertures forpassing electron beams and having long and short axes respectivelycorresponding to the long and short axes of the face panel, and firstand second halves which are symmetric with the short axis, and anon-aperture portion located around a periphery of the effectivesurface; the apertures being disposed so as to constitute a plurality ofaperture rows extending in parallel with the shots axis and disposed ina direction of the long axis, each of the aperture rows including aplurality of the apertures disposed in a direction parallel to the shortaxis and bridges positioned between any adjacent pair of the apertures;wherein a width B (x,y) of the bridges at a given coordinate position onthe effective surface is formed to be a size expressed by aquaternary-exponential polynomial as follows: ##EQU4## where the longaxis of the effective surface of the shadow mask is an x-axis, the shortaxis thereof is a y-axis, and c is a coefficient.